When I return to my download page I see an INVALID error message

After payment is made in Paypal you should be returned to the download page displayed by Linklok Paypal. If instead you are seeing an error message that looks something like this

Transaction not authenticated. payment_status= pending_reason= result=INVALID

then double check the following

1) Make sure auto return in your Paypal account is set to call

http://www.yoursite.com/linklokipnret.php

and not linklokipn.php which is for IPN only

2) Check that PDT in your Paypal account is enabled and the correct PDT token value set in $PDTtoken in linklokipn.php

3) Check that your Paypal account is a premier or business account as personal accounts don't allow website sales. Also check that the account is verified as unverified accounts often have PDT and IPN disabled.